1. Field of the Invention
This invention pertains to a mold gate design for encapsulating leaded electronic ceramics, ceramic resistor networks or other fragile electronic articles, hereinafter referred to as substrates, in thermoplastic material by means of insert molding. Insert molding is a plastic molding technique which enables the partial or complete covering of an article with plastic to produce a desired shape and/or dimension. In the molding process the article to be encapsulated is clamped in the mold cavity and then molten plastic is injected into the cavity under pressure to surround the article in the desired way.
2. Description of the Prior Art
It is common now in the electronics industry to encapsulate electronic devices in thermoset plastic material. The primary advantage is that thermoset plastic materials possess an ease of processing due to an intrinsically lower viscosity than thermoplastics. The lower viscosity permits lower injection pressures and slower injection rates, thus not causing the electronic ceramics to be broken. However, these advantages of thermoset plastic may be overcome with the present invention.
There are a number of reasons for wanting to encapsulate electronic devices in thermoplastic material rather than in thermoset plastic material. Thermoplastic materials are more economical than thermoset plastic materials. There is little or no waste with thermoplastics because they may be recycled, whereas thermoset materials cannot be recycled. The processing times for thermoplastics are relatively short, about 20 seconds, while thermosets require 2 to 3 minutes of process time. Because of the faster process time, thermoplastic molds can be smaller (having fewer cavities) for equal production rates with thermoset plastic materials. Thermoplastics also cause less internal stress on an encapsulated device because they do not adhere to the device as thermosets do. Examples of thermoplastic materials are polybutylene terephthalate, polyethylene terephthalate, and polyphenylene sulfide. Examples of thermoset plastic are diallyl phthalate or phenolic resins.
Two problems have been encountered when attempting to encapsulate fragile electronic ceramic substrates in thermoplastics. The first problem is one of controlling the position of the article in the mold cavity so that when the plastic is injected into the mold cavity, the device is completely encapsulated and centered in the plastic. The second problem is to inject the plastic into the mold cavity in such a way that it does not create unbalanced forces which will cause the device to be broken or distorted.
Several methods of encapsulating fragile electronic devices in thermoplastic have not been successful when tried, nor have successful methods been described in the literature. Mechanical means of prepositioning the electronic device and subsequently removing the positioning mechanism either prior to or simultaneously with the injection of the thermoplastic through other types of gate orifices have resulted in excessive device breakage or incomplete encapsulation due to the forcing of the device to one side of the mold cavity. Unsuccessful gate designs include a single large rectangular gate orifice located at the parting line of the mold cavity; a single round gate orifice split by a metal insert located in the nozzle of the injection machine; and two dual gate designs with one orifice located in the top half of the gate and one orifice located in the bottom half of the gate. These gates orifices were all located at the center line of the gate and mold cavity. Additionally, these gate designs were tried with and without the aid of positioning pins used to locate the device to be encapsulated in the center of the mold cavity. With each of these gate designs, excessive numbers of parts were either broken in molding or incompletely encapsulated as a result of being moved to the edge of the cavity during the injection of the plastic.